A film forming process, one of the semiconductor manufacturing processes, is performed by ALD (Atomic Layer Deposition) in which a raw material gas and a reactant gas for oxidizing, nitriding or reducing the raw material gas are alternately supplied, CVD (Chemical Vapor Deposition) in which a raw material gas is decomposed in a vapor phase or made to react with a reactant gas, or the like. As for the raw material gas used in the film forming process, a sublimated gas of the raw material is used in order to extremely reduce the amount of impurities introduced into a substrate and increase a density of crystal after film formation. For example, the raw material gas is used for a film forming apparatus for forming a high dielectric film by ALD.
Recently, along with the trend toward miniaturization of a wiring pattern formed on a semiconductor wafer (hereinafter, referred to as “wafer”), there is required a method capable of stabilizing a film thickness or a film quality and stabilizing a flow rate of a raw material. In supplying the raw material vaporized in a raw material container together with a carrier gas to a film forming unit, the amount of the vaporized raw material is changed depending on the filling amount of the raw material in the raw material container and, thus, the flow rate of the raw material is changed. Therefore, it is required to accurately check the remaining amount of the raw material in the raw material container and stabilize the supply amount of the raw material.
The replacement of the raw material container requires a long period of time. Thus, it is preferable to replace the raw material container to avoid emptying the raw material container during processing of wafers of a lot. For example, it is preferable to replace the raw material container during maintenance of a film forming apparatus. In order to prevent the operation stop of the film forming apparatus during the processing of the wafers of the lot, the raw material container needs to be replaced before the raw material container becomes empty. When it is difficult to check the accurate remaining amount of the raw material in the raw material container, a large margin is required and, thus, a large amount of the raw material is wasted during the replacement of the raw material container.
There is known a method for performing measurement by using a load cell or the like without separating the raw material container from the apparatus when measuring the remaining amount of the raw material in the raw material container. However, in the case of using as a raw material gas a gas obtained by sublimating a solid raw material, the raw material gas is easily resolidified when a temperature decreases and, thus, a line or the raw material container needs to be heated by a heating unit. Accordingly, the raw material container is fixed to a raw material gas supply unit, which makes it difficult to employ the method for measuring a weight by using a load cell.
Japanese Patent Application Publication No. 2012-234860 discloses a technique in which, with respect to a liquid raw material, a flow rate of a carrier gas supplied to a raw material container is adjusted to control a pressure in the raw material container and control a concentration of a raw material in a raw material gas to a constant level, and the amount of the raw material taken out of the raw material container is measured based on the concentration of the raw material in the raw material gas and a total flow rate of the raw material gas flowing through a raw material gas supply line. At this time, the amount of the raw material is calculated from the total flow rate of the raw material gas flowing through the raw material gas supply line and the flow rate of the carrier gas. However, there are variations between a flow rate controller for controlling the total flow rate and a flow rate controller for controlling the flow rate of the carrier gas. Therefore, when the remaining amount of the raw material is measured by adding the amount of the raw material taken out of the raw material container, the difference in the measurement values which is caused by individual variations between the devices becomes a problem.